2006 2500 whistle from rear end
 

I recently purchased a 2006 Chevy 2500 4x4 with 6.0 Vortec. I am getting a whistle out of the rear of the truck only in 1st gear. The noise seems to go away just before shifting. I changed rear diff fluid(was black) and checked the transmission fluid(good red). I took it to 2 different shops. 1 says its the rear end needs rebuit, the other says they need to tear down my transmission (this is from a tranny shop). The tuck has 116000 miles on it. Hoping someone has heard of something like this before. Thanks for any help you can provide.
Thanks,
Nick